Titanium alloys, aluminum alloys, and other metals commonly are used in aircraft skin and support structures because of their relatively light weight, high absolute strength, and high strength-to-weight ratio. To achieve desired physical properties, the alloys often are heat treated, which produces a dense, tightly adherent oxide in the form of scale or alpha case (or both) on outer surface. This oxide typically ranges in thickness from about 0.0001 to about 0.010 inches. It must be removed before subsequent machining, forming, or joining operations. Scale covered parts cannot be welded. Alpha case is difficult to machine and causes excessive tool wear or tool breakage. Also, alpha case can be a point source for cracking that may result in catastrophic failure.
Commonly today, the oxide is removed through chemical milling or etching of the metal in a series of chemical baths of highly toxic and corrosive concentrated alkaline and acid, including mixtures of nitric acid and hydrofluoric acid. Aerospace approved etching processes of this type are described in Boeing Process Specifications BAC 5753 "Cleaning, Descaling, and Surface Preparation of Titanium and Titanium Alloys" and BAC 5842 "Chemical Milling of Titanium." As a consequence, the baths and ancillary equipment that come into contact with these corrosive chemicals must be fabricated from expensive exotic materials that are resistant to attack.
To remove the surface oxide, then, the metal typically is immersed sequentially in acid baths for a period of time estimated to dissolve the scale without causing significant intergranular attack on the underlying titanium alloy substrate. Overimmersion results in undesirable intergranular attack. Underimmersion leaves scale or alpha case on the surface. On a single part, both underimmersion and overimmersion can occur at different locations on the surface. Either condition (i.e., intergranular attack or failure to remove surface oxides) can leave crack initiation sites (a "point source") for catastrophic failure of the part through cracking. Adjusting the chemical milling etch rate requires constant monitoring of the baths and frequent replenishment of the solution's constituents (i.e., the reagent's) of reagents. Orientation of the part in the bath effects the etch rate. Hydrogen generated during acid-etching may also migrate into the alloy structure causing "hydrogen-embrittlement"--a serious problem that reduces fatigue strength significantly. To avoid hydrogen embrittlement, treated parts usually are baked to remove the hydrogen.
Chemical milling generates a hazardous wastewater containing heavy metal ions that must be disposed of in an environmentally acceptable manner. Such disposal is becoming increasingly costly. The costs are detailed in U.S. patent application Ser. No. 08/522,644, filed Sep. 1, 1995 entitled "Removing Heavy Metals from Industrial Wastewater", now abandoned.